Unoriented surface-protection film of polypropylene block copolymers

ABSTRACT

A self-adhesive protective film having a backing layer and an adhesive layer, characterized in thatthe backing layer is an undrawn film,the backing layer comprises at least one polypropylene block copolymer andthe amount of polypropylene block copolymer makes up from 10 to 95% (w/w) of the protective film.

The invention relates to a weathering-stable self-adhesive protectivefilm with high adhesion for protecting surfaces such as glass, ceramic,stainless steel, polycarbonate glass of acrylic glass, especiallypainted surfaces. The protective films of the invention are particularlysuitable for protecting the fresh finish of vehicles such as automobilesand for protecting freshly painted vehicle parts against soiling anddamage during assembly, transit and storage.

For surface protection on unpainted areas there are numerous products.Painted areas have to date been covered little with self-adhesiveprotective films, and virtually not at all in the fresh state withoutbeing fully cured. When conventional protective films are used on freshpaint, numerous deficiencies arise, such as adhesive residues, instancesof discoloration, severe peel increase (increase in bond strengthfollowing storage in the bonded state), tearing on removal, shrinking onstorage, weak initial adhesion or low stretchability (especially onuneven bond substrates).

After prolonged periods, particularly after weathering, such effectsbecome intensified. Application on vehicles (automobiles) and vehicleparts (bumpers for example) where the surface is to be protected duringassembly or transit is therefore a challenging problem. The preservationand protection of motor vehicles during transit from manufacturer todealer has long been common practice.

For the preservation of automobiles it is known to apply paraffin waxesin a thickness of from 5 to 20 μm. However, it has been found that,especially on horizontal areas of the vehicles, a thin and usuallynon-uniform layer of this kind does not afford adequate protectionagainst external influences, such as the corrosive effect of birddroppings, for example.

A considerable disadvantage of paraffin wax sealing is the need toremove the preservative using a steam jet, surfactants or solvents.Environmentally sound recovery and disposal of the residues are a causeof great complexity in terms of apparatus and of very high costs.

Reversibly adhering self-adhesive films for protecting the automobileduring transit have likewise been known for some time but have so farbeen used only to a small extent. In many cases the films in questionare permanently adhering protective and decorative films, such as ananti-stonechip film, for example. These self-adhesive films remainpermanently on the vehicle after they have been mounted.

Adhesive tapes or protective films with self-adhesive compositions basedon natural rubber possess relatively good initial adhesion and may beremoved again without residue. Even on short-term exposure to UVradiation, however, these compositions are not stable to aging.Following extreme weathering exposure over a prolonged period (three tosix months), this leads to severe greasy residues or hardened paint-likeresidues on the painted metal.

Polyacrylate compositions, on the other hand, are very UV-stable. Ifuncrosslinked polyacrylate compositions are subjected to storage underalternating climatic conditions, their compatibility with paint surfacesis good only in some cases. Compatibility means that there are nodefects visible on the surface after the adhesive tape has been removed.On the other hand, however, these compositions exhibit severe peelincrease, so that when the film is difficult to peel off they leavesignificant residues of the adhesive composition on the substrate Ifthese compositions are strongly crosslinked chemically or by radiation,they may in some cases be removed without residue but on the other handcause permanent deformations on the paint surface and, furthermore, insome cases exhibit inadequate adhesion when bonded to curved paintedareas.

JP 02 199 184 discloses a radiation-crosslinkable pressure-sensitiveadhesive in which for protective application on paint films it isnecessary to formulate a low glass transition temperature in order toprevent permanent deformations of the paint film. Here, problems ariseowing to inadequate bond strength of the self-adhesive film to the motorvehicles during application and transit.

The same applies to acrylates as described in U.S. Pat. No. 5,612,136.Moreover, the method of irradiation disclosed therein damages the film,thereby reducing its weathering stability if irradiation is continued tothe point where the residual monomers have abated completely as a resultof polymerization. At relatively low irradiation doses, the residualmonomer content is so high that these monomers migrate into the vehiclefinish and, as a result, cause it to swell. A further disadvantage isthe migration of photoinitiators into the paint, where they lower its UVstability.

Self-adhesive compositions based on polyisobutylene (polyisobutylenehomopolymer or butyl rubber) exhibit easy removal in the bonding test inwhich they are applied to painted metal panels and then undergo storageunder alternating climatic conditions. The initial adhesion, however, islow, so that critical edges have to be fastened using a more stronglyadhering adhesive tape (a so-called edge securing tape). Under theinfluence of moisture, in particular the bond strength of the film isfrequently reduced to such an extent that the film detaches from theprotected vehicles in the course of transit, with the consequence thatthe protective effect is no longer there. Moreover, these adhesives aresoft (not very cohesive) and therefore produce residues of compositionwhen the film is removed, especially in the edge region after UV aging.Furthermore, this self-adhesive composition is not compatible with therubber seals (sealing profiles) customary in automobile construction orwith the plasticizers they contain; when the protective film is removedfrom window profiles, residues of the adhesive coating remain on therubber. Adhesive tapes and films of this kind are described in EP 0 519278, JP 95-325285 and U.S. Pat. No. 5,601,917.

EP 0 592 913 describes a surface protection film for the transit ofautomobiles which consists of an EVAc copolymer having a vinyl acetatecontent of 33%. With (his protective film, the application of anadhesive layer does not take place. Admittedly, a film of this kind hasa simple structure, but exhibits distinct disadvantages in use, Owing tothe high crystalline fraction the bond strength is very low; thisdeficiency is said to be eliminated by heating on application.Additionally these films, unless provided with a release cover (siliconepaper, for example), lead to blocking on storage, as taught by EP 0 768356. The films described exhibit severe paint deformation and are verydifficult to remove following use. A reason for the latter phenomenon isthat the EVAc described in the examples, with 33% (w/w) vinyl acetatehas sealing properties (it is commonly employed for sealingapplications). From 40% (w/w) vinyl acetate on, the crystalline fractionis so low that this defect does not occur; therefore, the protectivefilm of the invention also preferably comprises an adhesive having ahigh vinyl acetate content.

The adhesive sheet set out in DE 195 32 220, comprising EVAc adhesive,is significantly superior in adhesion to the products described. Withthe exception of one-component PU systems, there is no effect on thefinishes that are to be protected, During removal after use, however,this adhesive sheet has a high bond strength, and so cannot be removedwithout tearing. This increase in bond strength when the adhesiveassembly is stored, which the skilled worker calls peel increase, iscaused by interactions of the adhesive, especially polar forces, withthe paint. On the other hand, polar comonomers such as acrylate or vinylacetate give rise to the desired high initial adhesion.

Significantly more stable to UV than polyisobutylenes are adhesivescomprising hydrogenated styrene-diene block copolymers, whose use isdescribed in the utility model DE 296 04 473. A substantial disadvantageof such block copolymers is their thermally reversible crosslinking byway of the styrene domains. If an adhesive sheet produced from them isremoved in the summer from a vehicle which has become hot in the sun,the adhesive largely remains adhering to the paint, since the cohesionof the adhesive compositions is lower than the adhesion to the paint.Heating always leads to the shrinkage of the protective sheet; theadhesive, soft in the heat, smears onto the metal from the retreatingedges of the adhesive sheet.

EP 0 661 364 describes a surface protection film having an adhesivecomposition which is said to display good initial adhesion. Bondstrengths on steel of up to 1 kg/25 mm are described, correspondingapproximately to AFERA bond strengths to steel of up to 4.0 N/cm andresulting in particularly high adhesion following storage of theassembly. The films described are not stable to light, since no measuresare taken such as the addition of HALS light stabilizers orlight-reflecting pigments for achieving the light stabilizationnecessary for this application. The use of protective film on theoutside of vehicles imposes particularly extreme challenges in respectof light stability, especially in the UV region, so that despite goodinitial adhesion such protective films are entirely unsuitable for thisapplication. In the case of applications on vehicles, the stretchabilityof the film plays an important part owing to curved surfaces; however,the cited invention specifically emphasizes suitability for planar areas(plates and boards) and therefore also does not attempt to limit theforce at 10% elongation in order to achieve sufficient stretchability.

WO 96137568 describes the use of polyhexene and, respectively,polyoctene for a non-polar pressure-sensitive adhesive. Because of thelow cohesion, the polymers described in the examples do in fact exhibitlittle peel increase, but these polymers, owing to the low molecularweight of commercial polymers of this kind, likewise lead to residues,which attempts are made to avoid by adding other polymers, referred totherein as “Cold flow restricting agents”. For practical purposes,nevertheless, the specified adhesives are still of inadequate cohesion(softness), which leads after weathering to residues, especially if theadhesive tape shrinks under the affect of heat.

It is accordingly an object of the invention to provide an adhesive filmwhich possesses a sufficiently high initial bond strength so that whenbonded under tension (application of the adhesion film to curved areas)no detachment occurs and which nevertheless is easy to remove followingprolonged storage (up to one year under weathering). At the same time,no shrinkage should occur, and the protective film should be detachablewithout tearing and without sealing. Additionally, no release coveringshould be needed for storage.

This object is achieved by means of a protective film as specified inthe main claim. The subclaims relate to advantageous developments of thesubject matter of the invention. Furthermore, the invention alsoembraces particular fields of use of the protective film of theinvention, and a process for producing it.

The inventive subject matter relates accordingly to a self-adhesiveprotective film (also called adhesive surface protection tape) whichcomprises a layer of an undrawn backing layer (also called backing orbacking film) and an adhesive layer.

In advantageous embodiments of the invention, further layers such asadhesion promoter or release layers may be present additionally or aspart of the abovementioned layers.

As the backing layer of the adhesive, a thermoplastic polyolefin film isused which is unoriented and comprises at least one polypropylene blockcopolymer. The polypropylene block copolymer content of the protectivefilm is from 10 to 95% (w/w).

The UV transmittance of the protective film in the range from 290 to 360nm is preferably below 1%, more preferably below 0.1%. Additionally, thebond strength to steel is preferably between 0.2 and 3.8 N/cm, inparticular between 0.7 and 1.7 N/cm

Films of this kind may be manufactured on film blowing lines or,preferably, casting lines (T-die technology), with the film not beingmonoaxially or biaxially oriented by drawing (orientation) withstretching rolls or stretching frames. In the blowing of such a film,the orientation should be minimized by way of take-off speed, blow-upratio and temperature profile.

The polypropylene block copolymers used in accordance With the invention(and also called impact resistant polypropylene) are described in theliterature in Encycl. Polym. Sci. Technol. 13, 479ff (1988) and inUllmann's Encyclopedia of Industrial Chemistry A21, 529ff (1992).Examples of trade names are Propathene GSF 113 (ICI), 411 GA 05 (Amoco),PMA 6100 (Montell), Stamylan P (DSM), BD 801 F (Borealis), DaplenFFC1012 (PC), Novolen 2309 L.

Block copolymers of this kind differ from one another substantially intheir melt index (=MFI=MFR) and in the comonomer content. The melt indexinfluences the strength of the film and the fluidity of the melt inopposing directions. For the preparation of the protective film of theinvention, a melt index of from 0.8 to 15 g/l 0 min (ISO 1133 (A/4) at230° C. and 2.16 kg) is advantageous in order to achieve therequirements of toughness and tensile strength on the one hand andprocessibility (production speed and uniformity of thickness in the caseof coextruded films) on the other. The preferred range lies between 4and 10 g/10 min. With this product, coextrusion is an appropriate meansof introducing the adhesion promoter layer during the production of thefilm.

Where backing film and adhesive are connected to one another bycoextrusion, the selection of the melt index of the polypropylene blockcopolymer and of the other thermoplastic ingredients of the filmformulation is important. The amount of comonomer in polypropylene blockcopolymers determines the softness, tensile impact strength and heatstability of the protective film produced from them. The protective filmof the invention preferably comprises a polypropylene block copolymercontaining from 3 to 15% (w/w) ethylene as comonomer. The tensile impactstrength to DIN 53448 should be at least 1,000 mJ/mm² in both thelongitudinal (machine direction, MD) and transverse (cross-direction,CD) directions.

As further ingredients it is possible, for example, to use polyethylene(such as HDPE, LDPE, MDPE, LLDPE, VLLDPLE for example), copolymers ofethylene or propylene with polar comonomers, polypropylene homopolymersor polypropylene random copolymers to fine-tune the properties(mechanical, thermal or other properties such as gloss, adhesion of theadhesive, extrusion characteristics, etc.). It is particularlyadvantageous to combine two or more polypropylenes, especially ofdifferent softness and different melt indices, such as soft blockcopolymer with PP homopolymer or a hard block copolymer grade, forexample, since toughness, heat stability and flow characteristics can beadapted more effectively to the requirements than when a block copolymeralone is used. For sufficient heat stability, the fraction of propylenein a film layer should be at least 65% (w/w). With a multi-ply structureof the film layer, this is the layer which is responsible for thestrength and which therefore contains the highest polypropylene fraction(and in general also has the greatest thickness), and not any adhesionpromoter layer.

Preferred thicknesses for the backing films are from 20 to 80 μm(including the adhesion promoter layer where appropriate). During theapplication of the protective film, the softness of the backing filmplays a part in connection with the deformability; the force at 10%elongation should not exceed 25 N/15 mm, preferably 16 N/15 mm, ineither the machine or cross directions (tensile test to DIN 53455-7-5).This is also one of the reasons why the backing films should beunoriented. Stretching (also called drawing) raises the force at 10%elongation so greatly that the film is no longer assured ofconformability (for comparison, typical values are 100 N/15 mm formonoaxially oriented PP block copolymer and 180 N/15 mm for biaxiallyoriented homopolymer).

Furthermore, the strength in the transverse direction with respect toorientation or in the third dimension (perpendicular to the filmsurface) is so greatly reduced that when the protective film of theinvention is removed there is a danger of tears or delamination in thethird dimension (also called splitting). This applies in particular withadhesives possessing particularly good adhesion, such as, for example,the preferred ethylene-vinyl acetate copolymer adhesives. In thisrespect as well the use of polypropylene block copolymer has proven tobe particularly suitable, so that when an effectively adhering adhesiveis used the protective film does not tear on removal. In the course of atest of surface protection tapes following UV weathering it was foundthat, in the case of backing film made from standard polypropylene(polypropylene homopolymer), standard polyethylene (HDPE) or a 1:1mixture thereof, embrittlement occurred very quickly; in the case ofLDPE or polypropylene random copolymer (with 5% ethylene) the conditionsare more favorable, while the best results are achieved withpolypropylene block copolymer containing from 3 to 15% (w/w) ethylene.The advantages of using polypropylene block copolymer for the backingfilm therefore stand out particularly well in the case of removal afterweathering, which is highly relevant to practical use. With thisinvention's preferred combination of polypropylene block copolymer forthe backing film and strongly adhering ethylene-vinyl acetate copolymerfor the adhesive, it is possible to achieve the important properties ofhigh adhesion and yet good redetachability of the protective film in animpressive way without contradiction.

A further advantage was found to be the resistance of the protectivefilm of the invention to shrinkage under hot conditions. This isimportant in order that no greasy traces of the adhesive are formed atthe edges of the surface covered with the protective film. It is also ofadvantage for the passage of the film web through an oven for thepurpose of drying the adhesive. Protective films comprising polyethyleneas their main ingredient lack sufficient heat stability, even though theconformability (stretchability) is particularly advantageous forautomotive application.

The use of polyethylene or of polypropylene random copolymers as themain film constituent of surface protection tapes is customary onaccount of the fact that the conformability (see above undersoftness/deformability) is favorable. Additionally, the toughness isbetter than with polypropylene (homopolyrner), which has good heatresistance but is less brittle. The same applies to protective films forcovering automobiles. Protective films of this kind have thedisadvantage, however, that the heat stability is only moderate, The useof polypropylene block copolymers as the main film ingredient of thesurface protection tapes of the invention uniquely resolves the conflictbetween adequate softness on the one hand and heat stability on theother. The same applies to the contradictory requirement for hightoughness combined with heat stability.

The protective films of the invention preferably have a shrinkage inmachine and cross directions of less than 3%, with particular preferenceof less than 1%. The shrinkage is measured in a forced air oven at 120°C., by placing the sample on a heavily talced paper board for 10minutes.

A further surprising fact is that, as a substantial film ingredient,polypropylene block copolymer has a favorable effect on the unwindingcharacteristics. Protective films of this kind are generally used with awidth of more than 1 m. Under these conditions the unwinding forcebecomes relatively high, especially in the presence of strongly adheringadhesives, With the protective film of the invention it is thereforegenerally possible to dispense with a release coating for greater easeif unwinding. Following storage of the rolls of the surface protectiontape (protective film) with strongly adhering ethylene-vinyl acetatecopolymer adhesive, unwinding is fairly difficult if use is made neitherof a release nor of a polypropylene block copolymer as a substantialfilm ingredient. Accordingly, this combination is particularlyfavorable. Unwinding difficulties lead not only to overexertion on thepart of the persons using the film but also to an irreversibledistortion of the protective film at those points grasped by the person(or a gripper device) in order to unwind the roll.

As adhesive compositions it is possible to use elastomers such as, forexample, EPDM or EPM rubber, polyisobutylene, butyl rubber, EVM(ethylene-vinyl acetate copolymer), hydrogenated block copolymers ofdienes (for example by hydrogenation of SBR, cSBR, BAN, NBR, SBS, SIS orIR; such polymers are known, for example, as SEPS and SEBS) or acryliccopolymers such as ACM. Since the protective films of the invention areto be stable with respect to aging and weathering, unstable unsaturatedelastomers such as natural rubber, SIS, SBS, SBR or NBR are lesssuitable The light stabilizers specified for this film may also be usedfor the adhesive composition. This addition enhances the aging stabilityof the adhesive composition, especially with UV exposure above 100 kLy(kilolangleys), and is indispensable for applications involvingethylene-vinyl acetate copolymer adhesives with outdoor weathering forup to 6 months. In the case of adhesives comprising polyisobutylenehornopolymers or butyl rubbers, an addition of this kind is mandatory ifadhesive residues are to be avoided under very severe UV exposure.

It may be advantageous to crosslink the adhesive composition. Thisraises the heat stability (for example, under alternating climaticconditions): peel increase on rough or nonpolar substrates such assynthetic rubber window seals is reduced as a result. In addition and inparticular, crosslinking enhances the shear strength. This isparticularly important in the context of protective film application,since it resists shrinkage of the protective film in the application.For crosslinking, the customary crosslinking agents are suitable.Preference is given to the technique of radiation crosslinking,particularly with UV and electron beams. In the case of UV radiation,the adhesive composition is irradiated from the adhesive compositionside. The radiation dose may be reduced through the use ofphotoinitiators or the addition of crosslinking promoters such as estersof allyl alcohol, of methacrylic acid or of acrylic acid to the adhesivecomposition.

Copolymers of ethylene or vinyl acetate, especially ethylene-vinylacetate copolymers having a VAc fraction of at least 40% (w/w), provedto be particularly suitable. They guarantee a high bond strength even incritical cases (adhesive-repelling paints or strongly curved bond areas)and possess high weathering stability even without the addition of lightstabilizers to the adhesive. Preference is given to a VAc fraction offrom 55 to 70% (w/w). The combination of unoriented polypropylene blockcopolymer as backing film and ethylene-vinyl acetate copolymer asadhesive is also favorable on account of the fact that the one materialexhibits little shrinkage and the other is very highly resistant (shearstrength) to shrinkage in the bonded state. The shear strength is sogood that it is generally possible to dispense with crosslinking.

In order to optimize the properties, especially the bondingcharacteristics to specific paints, the self-adhesive composition(adhesive) employed may be blended with one or more additives such astackifiers (resins), plasticizers, fillers, pigments, UV absorbers,light stabilizers, aging inhibitors, crosslinking agents or crosslinkingpromoters.

Examples of tackifiers are hydrocarbon resins (formed, for example, fromunsaturated C₅ or C₇ monomers), terpene phenolic resins, terpene resinsmade from raw materials such as α- or β-pinene, aromatic resins such ascoumarone-indene resins or resins of styrene or α-methylstyrene, such asrosin and its derivatives such as disproportionated, dimerized oresterified resins, it being possible to use glycols, glycerol orpentaerythritol, and also others as listed in Ullmann's Enzyklopädie dertechnischen Chemie, volume 12, pages 525 to 555 (4th edition), Weinheim.Particularly suitable are resins stable to aging, without olefinicdouble bonds, such as hydrogenated resins, for example.

Examples of suitable fillers and pigments are carbon black, titaniumdioxide, calcium carbonate, zinc carbonate, zinc oxide, silicates orsilica.

Suitable UV absorbers, light stabilizers and aging inhibitors for theadhesive compositions are the same as those listed later on below forthe stabilization of the film.

Examples of suitable plasticizers for additional use are aliphatic,cycloaliphatic and aromatic mineral oils, diesters or polyesters ofphthalic acid, trimellitic acid or adipic acid, liquid rubbers (forexample nitrile rubbers or polyisoprene rubbers), liquid polymers ofbutene and/or isobutene, acrylates, polyvinyl ethers, liquid resins andsoft resins based on the raw materials for tackifier resins, lanolin andother waxes, or liquid silicones.

Examples of crosslinking agents include phenolic resins or halogenatedphenolic resins, melamine resins and formaldehyde resins. Examples ofsuitable crosslinking promoters include maleimides, allyl esters such astriallyl cyanurate, and polyfunctional esters of acrylic and methacrylicacid.

For certain paint systems it is advantageous to add constituents of thepaints to the adhesive in order to reduce the migration of paintconstituents (for example fatty acid esters of glycerol, silicones orother plasticizers) from the paint into the adhesive composition. Paintconstituents of this kind are described in more detail in EP 0 763 584and U.S. Pat. No. 5,612,135.

Furthermore, it is advantageous for physical recycling and heat energyrecovery if the adhesive, and particularly the entire protective film,is halogen-free

To improve the adhesion between film and adhesive it is advantageous toapply an adhesion promoter layer. The particularly preferred solution isa backing film consisting of a base layer comprising polypropylene blockcopolymer and an adhesion promoter layer, The latter preferablycomprises polymers and/or components of base layer and/or adhesive. Withparticular preference the adhesion promoter comprises at least onepolymer containing at least one o-olefin and a polar comonomer. Examplesthereof include ionomer, ethylene-vinyl acetate copolymer,ethylene-(meth)acrylic acid copolymer or maleic anhydride-modifiedpolyolefins.

The adhesion promoter may be applied by coextrusion with the base filmor by coextrusion with base film and adhesive, or by coating onto thebase film.

One advantageous embodiment comprises applying the adhesion promotersimultaneously with the extrusion of the film, so that the resultingcoextrusion film need only be coated with the adhesive. Technically thebest solution for the protective film of the invention is thesimultaneous coextrusion of film and adhesive (including an adhesionpromoter layer where appropriate).

In order to give the backing film weathering stability, it is advisableto add light stabilizers. There function with regard to the use of theprotective film consists primarily in preventing the embrittlement ofthe backing film, in order to avoid problems when the adhesive surfaceprotection tape is removed again. In the case of a multi-ply backingfilm, the protection or stabilization relates in particular to therelatively thick base layer. Light stabilizers of this kind aredescribed in Gaechter and Müller, Taschenbuch der Kunststoff-Additive,Munich 1979, in Kirk-Othmer (3rd) 23, 615-627, in Encycl. Polym. Sci.Technol. 14, 125-148 and in Ullmann (4th) 8, 21, 15, 529, 676. Inparticular, HALS light stabilizers such as, for example, dimethylsuccinate polymer with 4-hydroxy-2,2,6,6-tetramethyl-1-piperidineethanol(CAS No 65447-77-0), bis(2,2,6,6-tetramethyl4-piperidinyl) sebacate (CASNo. 52829-07-9) orpoly[[6-[(1,1,3,3-tetramethylbutyl)amino]-1,3,5-triazine-2,4-diyll][[(2,2,6,6-tetramethyl-4-piperidyl)imino]hexamethylene[(2,2,6,6-tetramethyl4-piperi-dyl)imino]](CAS No. 70624-18-9) are suitable for the protective film of theinvention. The amount of light stabilizer should be at least 0.15%,preferably at least 0.30% (w/w), based on the backing film.

The use of antioxidants for the film (for example, Irganox 1010 ortrisnonylphenyl phosphite) is advantageous but not absolutely necessary.Further suitable UV absorbers, light stabilizers and aging inhibitorsare set out in EP 0 763 584.

An improvement in the light stability of backing film and adhesive isalso possible by other additions to the protective film, particularly tothe base layer of the backing film. This may be done by means of UVabsorbers (for example Tinuvin P, Ciba) or, primarily, by means ofreflecting pigments (for example titanium dioxide).

It may be appropriate to preformulate batches of film additives such astitanium dioxide, light stabilizers, aging inhibitors or procesasingaids, individually or together, preferably in polyethylene,polypropylene or polypropylene block copolymer.

The subject matter of the invention is especially suitable forapplication on painted vehicles such as automobiles. In this context, abond strength of at least 0.7 N/cm even in the freshly bonded state isdesirable (AFERA method 4001, corresponding to DIN EN 1939), in orderthat the film is unable to become detached at bond edges as a result ofthe stresses (especially in the case of uneven bonding) or even tobecome detached entirely as a result of the slipstream during transit ordriving of the automobile. Since the bond strength on paints depends ontheir respective paint formulation, the bond strength of the protectivefilm may be characterized more effectively by means of the bond strengthto steel (AFERA). In the freshly bonded state this strength should asfar as possible be between 0.5 and 3.8 N/cm, preferably between 0.7 and1.7 N/cm.

The advantages of the unoriented polypropylene block copolymer for anautomotive protective film have already been set out. Virtually allattempts to produce adhesive, adhesion promoter and film (comprising theblock copolymer) by coextrusion in a single step have failed to date,since at a width of more than 1 m it was impossible to achieve uniformlayer thickness distribution. The repetition of examples known from theliterature for preparing protective films of the general kind bycoextrusion shows that it was not possible to achieveapplication-compatible layer thickness distribution if the width of theunit is more than 1 m. This applies in particular when using rawmaterials for strongly adhering adhesives, i.e., not only slightlytacky, partially crystalline copolymers such as are commonly used forsealing layers. It was therefore very surprising that ethylene-vinylacetate copolymers containing at least 40% (w/w) vinyl acetate withblock copolymers could be coextruded as described in the example. Thelayer thickness distribution is so good that, with a running length of1000 m and a width of 1.5 m, uniformly wound rolls were formed and thebond strength to steel (AFERA 4001) across the width lies within atolerance range of ±20%. In order to achieve this result, it isnecessary to harmonize the melt indices of the raw materials of thelayers and the temperatures of the melt flows.

The most advantageous ranges are the following:

The melt indices of the film (including component layers such asprimers) are from 1 to 10 g/10 min at 230° C. and 2.16 kg and, for theadhesive, from 0.5 to 10 g/10 min at 190° C. and 2.16 kg. Thetemperature of the adhesive melt lies below that of the film (measuredat the point where the melt flows unite).

Surprisingly and unforeseeably for the person skilled in the art, aprotective film having a backing layer of undrawn film based on at leastone polypropylene block copolymer and, where appropriate, lightstabilizer in an amount of at least 0.15% (w/w), with a polypropyleneblock copolymer content of from 10 to 95% (w/w) in the protective filmat a preferred UV transmittance in the range from 290 to 360 nm of below1% and a bond strength to steel of between 0.2 and 3.8 N/cm, especiallywhen using ethylene-vinyl acetate copolymer containing at least 40%(w/w) vinyl acetate, exhibits the desired combination of properties:

low shrinkage (high heat stability)

good UV resistance

high toughness (protection against mechanical attack)

good stretchability

sufficient bond strength on application to curved surfaces

no residues when the protective film is removed

The EVAc self-adhesive compositions used possess, on the one hand, goodadhesion to a variety of finishes common in the automotive industry,which is retained under the effect of moisture or humid conditions, sothat the protective film does not detach from the vehicle even underwind exposure or under tension caused by bonding to curved surfaces.Moreover, the self-adhesive composition possesses a sufficient bondstrength within the first few minutes after application, so that afterjust half an hour, for example, the protective film may be subjected tosevere slipstream load (up to 160 km/h), but on the other hand may alsobe removed without tearing following prolonged use. Furthermore, evenwithout application of a release layer, the protective film of theinvention has an unwind force which is sufficiently low for the user,despite the strongly adhering adhesive.

The peel force (bond strength) of the protective film of the inventionfrom (to) 2K PU paints is generally more then 0.7 in the fresh state andless than 4 N/cm following storage under alternating climatic conditions(in analogy to AFERA method 4001). Even exposure of the protective filmto UV light, using for example a Xenotest 1200 at 55° C. for 3000 hours,does not cause any deficiencies in the properties of the protectivefilm: there is no embrittlement of the film and there are no residues ofcomposition on removal.

The protective film of the invention is therefore particularly suitablefor protecting the fresh finish of automobiles during assembly ortransit or as processing and transit protection for freshly paintedsteel panels. The protective film can be bonded just half an hour afterthe painted surfaces have passed through the oven, without anydisadvantages whatsoever, despite the fact that at this point in timethe finish has not yet fully cured. The protective film also exhibitsoutstanding properties when used as an edge securing tape for theadditional fastening of other kinds of extensive self-adhesive coverfilms with low bond strength.

A further feature of the protective film of the invention is that it canbe applied in a large width over the hood, roof and trunk of automobilesand that, owing to its deformability, it conforms very well to planarand even gently curved shaped areas. It is therefore possible to protectthe horizontal areas which are most at risk from soiling. However, eventnarrow areas such as, for example, the protection of the door below thewindows, or bumpers, can easily be covered. Protection of the verticalareas on the vehicle is particularly appropriate during its assembly.Despite the good stretchability, the defect of shrinkage of conventionalreadily deformable polyethylene protective films is not in evidence.

The protective film is resistant to sunlight, moisture, heat and cold,with weathering stability of at least one year. In particular, theaddition of pigments such as titanium dioxide and of light stabilizersleads to an improvement in the UV stability of the protective film. Evenvery high sun levels, such as are encountered in Florida, for example,do not cause the protective film to fail or detach. The extremely low UVtransmittance of the protective film prevents the adhesive being brokenby sun exposure.

Furthermore, the strength of the protective film in comparison topreservation with wax ensures impeccable protection against soiling suchas bird droppings and against damage to the vehicle as a whole by minormechanical events. Despite the requisite effect of adhesion, theprotective film can be removed after use without residue and withouttearing of the backing film. It is possible to recycle the protectivefilm or recover energy from it, in particular since it is halogen-free.

In the text below, the invention will be illustrated on the basis ofexamples which are not, however, intended to restrict the invention.

EXAMPLES Example 1

A film was manufactured by flat film extrusion in a width of 1450 mm. Itwas composed of a 50 μn base layer and a 10 μm adhesion promoter layer.The base layer consisted of 91.3% (w/w) Novolen 2309 L block copolymer(BASF, melt index 6 g/10 min at 230° C. and 2.16 kg, ethylene contentapprox. 6.5% (w/w)), 8.4% (w/w) titanium dioxide and 0.3% (w/w) Tinuvin770 HALS stabilizer. The adhesion promoter layer was composed of 50%(w/w) Novolen 2309 L and 50% (w/w) Escorene Ultra UL 00728 EVAccopolymer (Exxon). Prior to extrusion, the raw materials were premixedusing a concrete mixer.

The film had the following physical properties:

Overall thickness of film with 60 μm DIN 53370 adhesion promoter Overallweight of film with 59 g/m² DIN 53365 adhesion promoter Tensilestrength, MD 45 N/mm² DIN 53455-7-5 Force at 10% elongation, MD 14.5N/cm DIN 53455-7-5 Tensile strength, CD 42 N/mm² DIN 53455-7-5Elongation, MD 820% DIN 53455-7-5 Elongation, CD 840% DIN 53455-7-5Shrinkage, MD  0.9% 10 min 120° C. Shirnkage, CD  0% 10 min 120° C.Tensile impact strength, MD >3000 mJ/mm² DIN 53448 Tensile impactstrength, CD 2500 mJ/mm² DIN 53448 UV transmittance  0.2%spectrophotometer

The adhesive used was a 10% strength solution of the following rawmaterials in toluene:

65% (w/w) of a copolymer of 70% (w/w) vinyl acetate and 30% (w/w)ethylene having a Mooney viscosity ML (1+4) at 100° C. (DIN 53523)without pretreatment of 28.

30% (w/w) of a copolymer of 45% (w/w) vinyl acetate and 55% (w/w)ethylene having a Mooney viscosity ML (1+4) at 100° C. (DIN 53523)without pretreatment of 18.

4.8% (w/w) of a hydrogenated rosin glycerol ester.

0.2% (w/w) of Irganox 1010 (antioxidant).

The adhesive solution was applied to the film using a coating bar anddried in a tunnel at 80° C. for four minutes. The resulting protectivefilm was edged and wound into rolls 200 m long and 1,400 mm wide. Theapplication of adhesive amounted to 20 μm.

The self-adhesive film produced in this way was readily unwindable,without creases, and could be applied flawlessly when used to protectautomobiles. The protective film was characterized by the physicalproperties shown in the following table.

Overall thickness of protective film 80 μm Bond strength to 2K PU paintafter 3 days at 90° C. with a 3.6 N/cm peel angle of 180° and a peelrate of 300 mm/min Bond strength to the reverse, at a peel angle of 180°and a 0.7 N/cm peel rate of 300 mm/min Bond strength to steel 1.0 N/cmBond strength to paint 0.9 N/cm

The protective film was bonded to metal panels which had been freshlypainted (with 2K PU paint) and was removed by peeling after temperaturestorage (3 days at 90° C.). In the edge region only slight paintdeformations were evident, while under oblique light a slight loss ofbrightness was observable over the area. No shrinkage was evident at thebond edges. Samples bonded to painted metal were subjected to UV aging(1750 h of Xenotest 150, corresponding to 97 kLy); following removalthere were neither residues of adhesive composition nor tears, and noshrinkage was evident at the bond edges. A curved auto hood withstrongly pronounced beads was covered with the adhesive protective film,with the aid of a felt doctor. There were no creases or holes owing toinadequate stretchability or detachment at the bond edges.

Example 2

Production was carried out as in example 1 but the adhesive used was amixture of polyisobutylenes of which 35% (w/w) had a molecular weight Mwof 1.2.10⁶ g/mol and 65% (w/w) an Mw of 35,000 g/mol. The bond strengthto paint (fresh) was 0.4 N/cm or 1.7 N/cm (after 3 days of storage at90° C.) and to steel it was 0.5 N/cm. The protective film was bonded tometal panels which had been freshly painted (with 2K PU paint) and wasremoved by peeling after temperature storage (3 days at 90° C.). In theedge region only slight paint deformations were evident, while underoblique light a slight loss of brightness was observable over the area.No shrinkage was evident at the bond edges, Samples bonded to paintedmetal were subjected to UV aging (1750 h of Xenotest 150, correspondingto 97 kLy); following removal there were neither residues of adhesivecomposition (with the exception of a narrow, 0.1 mm strip at the bondedges) nor tears, and no shrinkage was evident at the bond edges. Acurved auto hood with strongly pronounced beads was covered with theadhesive protective film, with the aid of a felt doctor. There were nocreases or holes owing to inadequate stretchability. The same protectivefilm was bonded to a 1 K PU paint on metal sample panels and on avehicle. The bond strength to paint was 0.2 N/cm (in the fresh state) or1.4 N/cm (after 3 days of storage at 90° C.). At the edges of areas onwhich the film was stretched to a relatively high degree, the protectivefilm lifted off to such an extent that it had to be reinforced using anedge securing tape.

Example 3

The protective film was produced by coextrusion of three layersincluding adhesive. Raw materials:

main layer 50 μm: 60% (w/w) Daplen FFC 2012 block copolymer (PCD, meltindex 5 g/10 min at 230° C. and 2.16 kg, ethylene content approx. 12%(w/w)), 25% (w/w) Daplen KF 201 homopolymer (PCD, melt index 8 g/10 minat 230° C. and 2.16 kg), 6.3% (w/w) Lupofen 1840 H (LDPE, Elenac GmbH,melt index 1.5 9/10 min at 190° C. and 2.16 kg, density 0.919 g/cm³),8.4% (w/w) titanium dioxide and 0.3% (w/w) Tinuvin 770.

adhesion promoter layer 10 μm: as example 1

adhesive layer 25 μm: copolymer of 60% (w/w) vinyl acetate and 40% (w/w)ethylene with a melt index of 3 g/10 min at 190° C. and 2.16 kg. Priorto extrusion, the adhesive base material was processed into granulesapproximately 5 mm in size, with the addition of about 1% (w/w) ofsilica as powdering agent.

The physical properties of the film were as follows:

Overall thickness of the protective film 85 μm DIN 53370 Overall weightof the protective film 83 g/m² DIN 53365 Tensile strength, MD 43 N/mm²DIN 53455-7-5 Force at 10% elongation, MD 12 N/cm DIN 53455-7-5 Tensilestrength, CD 37 N/mm² DIN 53455-7-5

Elongation, MD 790% DIN 53455-7-5 Elongation, CD  860%  DIN 53455-7-5Shrinkage, MD 0.5% 10 min 120° C. Shrinkage, CD 0.1% 10 min 120° C.Tensile impact strength, MD >3000 mJ/mm² DIN 53448 Tensile impactstrength, CD  2800 mJ/mm² DIN 53448 UV transmittance 0.2%spectrophotometer *calculated assuming a thickness of 60 μm (excludingadhesive layer).

The roll of protective film produced in this way was 1.5 m wide and 1000 m long, with impeccable quality of winding. The roll could beunwound easily and without creases and could be applied flawlessly whenused to protect automobiles. Following use, this self-adhesive filmcould be removed again by peeling without deficiencies after a bondingperiod of up to one year under outdoor weathering. The protective filmwas characterized by the physical properties shown in the followingtable.

Bond strength to 2K PU paint after 3 days at 90° C., with a 2.7 N/cmpeel angle of 180° and a peel rate of 300 mm/min Bond strength to thereverse, at a peel angle of 180° and a 0.4 N/cm peel rate of 300 mm/minBond strength to steel 1.1 N/cm Bond strength to paint 1.0 N/cm

The protective film was bonded to metal panels which had been freshlypainted (with 2K PU paint) and was removed by peeling after temperaturestorage (3 days at 90° C.). In the edge region no paint deformation wasevident. No shrinkage was evident at the bond edges. Samples bonded topainted metal were subjected to UV aging (1750 h of Xenotest 150,corresponding to 97 kLy); following removal there were neither residuesof adhesive composition nor tears, and no shrinkage was evident at thebond edges. A curved auto hood with strongly pronounced beads wascovered with the adhesive protective film, with the aid of a feltdoctor. There were no creases or holes owing to inadequatestretchability or detachment at the bond edges.

COMPARATIVE EXAMPLES Comparative Example 1

Production was carried out as in example 1. However, the film used was amonoaxially oriented film of Borealis P410F (melt index 0.9 g/10 min at230° C. and 2.16 kg, ethylene content approx. 9%). The film was producedas in comparative example A of DE 36 40 861 A1.

The film had the following physical properties:

Overall thickness of film with 63 μm DIN 53370 adhesion promoter Overallweight of film with 60 g/m² DIN 53365 adhesion promoter Tensilestrength, MD 350 N/mm² DIN 53455-7-5 Force at 10% elongation, MD 150N/15 mm DIN 53455-7-5 Elongation, MD 38% DIN 53455-7-5 Shrinkage, MD  8%10 min 120° C. Tensile impact strength, MD 1200 mJ/mm² DIN 53448 Tensileimpact strength, CD 250 mJ/mm² DIN 53448 UV transmittance 95%spectrophotometer

The roll could not be unwound because the film underwent splitting(tearing in the 3rd dimension). For bonding tests, therefore, sampleswere used which had been covered with silicone paper after coating. Theprotective film was characterized by the physical properties shown inthe following table.

Overall thickness of protective film: 83 μm Bond strength to 2K PU paintafter 3 days at 90° C. with film undergoes a peel angle of 180° and apeel rate of 300 mm/min splitting Bond strength to the reverse, at apeel angle of 180° and 1.2 N/cm a peel rate of 300 mm/min Bond strengthto steel 1.1 N/cm Bond strength to paint 1.0 N/cm

The protective film was bonded to metal panels which had been freshlypainted (with 2K PU paint). After 3 days at 90° C. it was removable onlywith splitting and tearing (in the longitudinal direction). No shrinkagewas evident at the bond edges longitudinally, while in thecross-direction the shrinkage was 5%. Samples bonded to painted metalwere subjected to UV aging (1750 h of Xenotest 150, corresponding to 97kLy); the protective film had broken down into crumbs. At the originalbond edges crosswise, severe shrinkage was still in evidence. A curvedauto hood with strongly pronounced beads was covered with the adhesiveprotective film, with the aid of a felt doctor. There were severecreases and tears in the longitudinal direction owing to inadequatestretchability.

Comparative Example 2

Production was as in example 1 but for the film a PP random copolymerNovolen 3300 MC (Targor GmbH) was used: melt index 8.0 g910 min at 230°C. and 2.16 kg, ethylene content approx. 3.5% (w/w).

The physical properties of the film were as follows:

Overall thickness of film with 60 μm DIN 53370 adhesion promoter Overallweight of film with 59 g/m² DIN 53365 adhesion promoter Tensilestrength, MD 35 N/mm² DIN 53455-7-5 Force at 10% elongation, MD 10 N/cmDIN 53455-7-5 Elongation, MD  610%  DIN 53455-7-5 Shrinkage, MD 3.1% 10min 120° C. Tensile impact strength, MD 2100 mJ/mm² DIN 53448 Tensileimpact strength, CD  800 mJ/mm² DIN 53448 UV transmittance 0.2%spectrophotometer

The self-adhesive film produced in this way was vaguely distorted at thepoints at which the film was held during unwinding. However, owing tothe flexibility, it could be applied effectively to a vehicle.

The protective film was bonded to metal panels which had been freshlypainted (with 2K PU paint) and was removed by peeling after 3 days at90° C. At all bond edges there was distinct shrinkage in evidence(residues of adhesive in a width of several mm). Samples bonded topainted metal were subjected to UV aging (1750 h of Xenotest 150,corresponding to 97 kLy); following removal there were no residues ofadhesive composition but there were several tears, and severe shrinkageoccurred at two bond edges. A curved auto hood with strongly pronouncedbeads was covered with the adhesive protective film, with the aid of afelt doctor. There were no creases or holes owing to inadequatestretchability or detachment at the bond edges.

Comparative Example 3

Production was as in example 1 but for the film a PP homopolymer Novolen1102 M (Targor GmbH) was used; melt index 8.0 g/10 min at 230° C. and2.16 kg, ethylene content zero. In addition, light stabilizers andtitanium dioxide were omitted.

The physical properties of the film were as follows:

Overall thickness to film with 60 μm DIN 53370 adhesion promoter Overallweight of film with 55 g/m² DIN 53365 adhesion promoter Tensilestrength, MD 50 N/mm² DIN 53455-7-5 Force at 10% elongation, MD 17 N/cmDIN 53455-7-5 Elongation, MD  640%  DIN 53455-7-5 Shrinkage, MD 0.5% 10min 120° C. Tensile impact strength, MD 2500 mJ/mm² DIN 53448 Tensileimpact strength, CD  200 mJ/mm² DIN 53448 UV transmittance  94%spectrophotometer

The protective film was bonded to metal panels which had been freshlypainted (with 2K PU paint) and was removed by peeling after 3 days at90° C. Shrinkage or tears were not observed. Samples bonded to paintedmetal were subjected to UV aging (1750 h of Xenotest 150, correspondingto 97 kLy). The film had broken down completely; however, it was stillpossible to recognize that no shrinkage had occurred. A curved auto hoodwith strongly pronounced beads was covered with the adhesive protectivefilm, with the aid of a felt doctor. There were a few creases but nodetachment at the bond edges.

Comparative Example 4

Production was carried out as in example 1, for the film, however, amixture of 37% (w/w) Novolen 1102 M PP homopolymer (Targor GmbH, meltindex 8.0 g/10 min at 230° C. and 2.16 kg), 54.3% (w/w) Lupolen 1840 HLDPE (Elenac GmbH, melt index 1.5 g/10 min at 190° C. and 2.16 kg), and8.4% (w/w) titanium dioxide and 0.3% (w/w) Tinuvin 770 HALS stabilizerwas used.

The film had the following physical properties:

Overall thickness of film with 68 μm DIN 53370 adhesion promoter Overallweight of film with 63 g/m² DIN 53365 adhesion promoter Tensilestrength, MD 30 N/mm² DIN 53455-7-5 Force at 10% elongation, MD 13 N/cmDIN 53455-7-5 Elongation, MD  720%  DIN 53455-7-5 Shrinkage MD 2.5% 10min 120 ° C. Tensile impact strength, MD 3200 mJ/mm² DIN 53448 Tensileimpact strength, CD  300 mJ/mm² DIN 53448 UV transmittance 0.2%spectrophotometer

The protective film was characterized by the physical properties shownin the following table.

Overall thickness of protective film: 81 μm Bond strength to 2K PU paintafter 3 days at 90° C. with 3.6 N/cm a peel angle of 180° and a peelrate of 300 mm/min Bond strength to the reverse, at a peel angle of 180°and 0.7 N/cm a peel rate of 300 mm/min Bond strength to steel 1.1 N/cmBond strength to paint 1.0 N/cm

The protective film was bonded to metal panels which had been freshlypainted (with 2K PU paint) and removed by peeling after 3 days at 90° C.Slight shrinkage and also residues of adhesive were evident at the bondedges. Slight paint deformation occurred at the edges, with creases.Samples bonded to painted metal were subjected to UV aging (1750 h ofXenotest 150, corresponding to 97 kLy). Following removal, there were noresidues of adhesive composition remaining. A curved auto hood withstrongly pronounced beads was covered with the adhesive protective film,with the aid of a felt doctor. A high level of creasing occurred. Ondemasking after 48 hours, the film tore in some cases in strips parallelto the longitudinal direction.

What is claimed is:
 1. A self-adhesive protective film having a backinglayer and an adhesive layer, wherein the backing layer is an undrawnfilm, the backing layer comprises at least one polypropylene blockcopolymer in an amount of from 10 to 95% by weight of the protectivefilm, said polypropylene block copolymer comprises ethylene and theethylene content of the block copolymer is between 3 and 15% (w/w). 2.The self-adhesive protective film according to claim 1, wherein the bondstrength to steel is between 0.2 and 3.8 N/cm.
 3. The self-adhesiveprotective film according to claim 1, wherein the tensile impactstrength is at least 1 000 mJ/mm² in both machine and cross directions.4. The self-adhesive protective film according to claim 1, wherein theshrinkage is less than 3% in both machine and cross directions.
 5. Theself-adhesive protective film according to claim 1, wherein the UVtransmittance in the range from 290 to 360 nm is below 1%.
 6. Theself-adhesive protective film according to claim 1, wherein the force at10% elongation does not exceed 24 N/15 mm in either the machine or crossdirections.
 7. The self-adhesive protective film according to claim 1,wherein the thickness of the backing layer is between 20 and 80 μm. 8.The self-adhesive protective film according to claim 1, wherein thefraction of propylene in one film is at least 65% (w/w).
 9. Theself-adhesive protective film according to claim 1, wherein the meltindex of the polypropylene block copolymer is between 0.8 and 15 g/10min at 230° C. and 2.16 kg.
 10. The self-adhesive protective filmaccording to claim 1, wherein the backing layer comprises at least onelight stabilizer in an amount of at least 0.15% (w/w).
 11. Theself-adhesive protective film according to claim 1, wherein the adhesivelayer comprises at least one copolymer of ethylene and vinyl acetate,with the vinyl acetate content of the adhesive layer being at least 40%(w/w).
 12. The self-adhesive protective film according to claim 1, whichcomprises an adhesion promoter layer between the backing layer and theadhesive composition.
 13. The self-adhesive protective film according toclaim 12, wherein the backing layer is coextruded together with theadhesion promoter layer.
 14. The self-adhesive protective film accordingto claim 1, wherein said self-adhesive protective film is produced bysimultaneous coextrusion of the adhesive composition and of the backingfilm.
 15. A method for protecting painted areas of vehicles or vehicleparts which comprises applying the self-adhesive protective film ofclaim 1 to said painted areas of said vehicles or vehicle parts.
 16. Theself-adhesive protective film of claim 2, wherein said bond strength isbetween 0.7 and 1.7 N/cm.
 17. The self-adhesive protective film of claim4, wherein said shrinkage is less than 1%.
 18. The self-adhesiveprotective film of claim 6, wherein said force at 10% elongation doesnot exceed 16 N/15 mm in either the machine or cross directions.
 19. Theself-adhesive protective film of claim 11, wherein said vinyl acetatecontent is from 55 to 70% by weight.
 20. The self-adhesive protectivefilm of claim 12, wherein the thickness of the backing film and adhesionpromoter layer is between 20 and 80 μm.